Wheel calculators have been provided heretofore in a wide variety of configurations and for many different purposes. For example, a circular slide rule may combine a disk with a cursor on a member rotatable relative to the disk and circular slide rules may have several disks, a window arrangement and various index markings enabling calculations to be made based upon indicia on the disks or the relatively rotatable members. Less sophisticated wheel calculators may include an information carrying wheel rotatable beneath or between other disk shaped members or cover structures which may be provided with windows or clear areas or openings through which information on the disk can be read. Wheel calculators may be used to forecast events, as calendars, as calculators for taxes, fees, rebates or the like, and as devices facilitating the selection of a particular condition. They may be free to hold in the hand, or permanently affixed into, say, a book, and may be themselves promotional materials or may be incorporated in promotional materials. In practically all cases in which a wheel has been a rigid member in earlier systems, that wheel was specifically designed for the calculator, generally had a considerable thickness, and was especially fabricated and hence of comparatively high cost.
Where the wheel was composed of a nonrigid material, e.g. paper or cardboard, it could be effectively die cut from a web or sheet of such material and provided with a central perforation or die cut to accommodate the member forming a journal for that wheel, i.e. a member enabling rotation of the wheel relative to something else.
Flexible wheels of this type were prone to damage and were easily distorted not only by misuse but even in regular or careful use.